This invention relates to projection lens systems for use in projection televisions and, in particular, to color corrected, wide field of view, high numerical aperture projection lens systems for use with cathode ray tubes (CRTs), including cathode ray tubes having curved faceplates.
Various color-corrected high image quality lenses for use in high definition TV displays (HDTV) and in the projection of data and graphics are known in the art. These lenses are most frequently used in xe2x80x9cfront screenxe2x80x9d two piece systems, i.e. systems where the projector and the screen are two different units. As a result of the long distance between the projector and the screen, most of the lenses used in such systems have a half field of view of under 30xc2x0.
In recent years, one piece projection TVs have become increasingly popular. These systems utilize a xe2x80x9crear screenxe2x80x9d configuration in which the image is projected onto the rear surface of a translucent screen which is combined with the projector into a single unit. To achieve a small overall size for such systems, the lens must have a field of view as wide as possible.
To help achieve this goal and to provide for an increased amount of light at the outer portions of the image, CRTs having curved faceplates are most often used in this application. The faceplates of such CRTs are plano-convex shaped with the phosphor being deposited onto the curved side of the faceplate. As a result, the outer portion of the phosphor side of the faceplate curves towards the lens.
Presenting the CRT image on a surface concave towards the projection lens allows the lens to achieve a half field of view in excess of 40xc2x0. However the control of electron beam spot size on a curved phosphor surface is much more difficult than on a flat surface. Spot size control is important since a small and well controlled spot size is required to produce a high quality image.
As long as spot size was fairly large, projection lenses did not need to be corrected for axial color. However, since the introduction of digital TV (e.g., satellite TV and DVD), the quality level of one piece rear projection TV sets for consumer use has been significantly raised.
Manufacturers of such systems are now more willing to use more complicated electronics to minimize and control the size of the spot on a curved phosphor surface, e.g., they are willing to produce spot sizes whose sizes are 0.15 millimeters or less. Consequently, new high quality wide field of view large aperture lenses are needed to compliment the higher quality outputs of curved phosphor CRTs. As with the optics used in data and graphics projection TV systems, these new lenses need to be corrected for color.
A typical color corrected lens used with a flat faceplate CRT consists from long conjugate to short of a front weak aspherical unit, a main power unit which includes a color correcting doublet and a strong positive element having most of the power of the lens, a corrector unit following the main power unit and having at least one aspherical surface, and a strong negative power unit associated with the CRT faceplate and providing most of the correction for the field curvature of the lens. See Kreitzer, U.S. Pat. No. 4,900,139.
From the image side, the main power unit typically has a negative element followed by a positive element of similar focal length but of opposite sign. These two elements provide color correction for the lens and their combined shape is typically meniscus towards the long conjugate. The single positive element providing most of the power of the lens usually follows the color correcting doublet.
Moskovich, U.S. Reissue Pat. No. 35,310, discloses color corrected projection lenses having three lens units wherein each of the first and second units has a positive low dispersion element and a negative high dispersion element.
Co-pending and commonly assigned U.S. patent application No. 09/005,916, filed Jan. 12, 1998, in the name of Jacob Moskovich and entitled xe2x80x9cColor Corrected Projection Lenses For Use With Curved Faceplate Cathode Ray Tubes,xe2x80x9d discloses projection lenses for use with curved CRTs wherein the second lens unit has two positive lens elements at least one of which is at the image side of the lens unit.
The foregoing approaches to achieve color correction have each employed at least one negative lens element of high dispersion which has meant that additional positive power had to be added to the system to compensate for the negative power of the negative element. The additional positive power has taken the form of stronger positive elements or, in many cases, the inclusion of an additional positive element in the system. The incorporation of additional positive and negative elements has increased the cost, complexity, and weight of the lens system. In particular, weight has been increased when the color correction has been achieved using glass elements. The use of glass elements has also meant working with flint glass for the negative high dispersion elements. As known in the art, flint glass is more difficult to work with than crown glass.
In view of the foregoing, it is an object of the present invention to provide a projection lens system which (1) has a large aperture, i.e., a f/number of about 1.2 or less, (2) has a wide field of view, i.e., a half field of view of at least 35xc2x0, (3) provides a high level of correction of both chromatic and monochromatic aberrations when used with cathode ray tubes, including cathode ray tubes having curved faceplates, and (4) achieves chromatic aberration correction with a minimum of additional lens elements and in some cases no additional lens elements.
To achieve these and other objects, the invention provides a projection lens system which from long conjugate to short comprises:
(A) a front lens unit (first lens unit; U1) comprising at least one aspherical element (i.e., an element having at least one aspherical surface), said front lens unit having a short conjugate side (S2 in Tables 1 and 2),
(B) a positive power lens unit (second lens unit; U2) which preferably provides the majority of the power of the lens system,
(C) a corrector lens unit (UCR) comprising at least one aspherical element (i.e., an element having at least one aspherical surface), and
(D) a strong negative power unit (third lens unit; U3) associated with the CRT faceplate having a strong concave surface (S11 in Tables 1 and 2) facing the long conjugate and providing most of the correction of the field curvature of the lens, said strong negative power unit having a long conjugate side (S11 in Tables 1 and 2),
wherein the lens system includes at least one diffractive optical surface (DOS) which at least partially corrects the axial color of the lens system and which is located between the short conjugate side of the front lens unit and the long conjugate side of the strong negative power unit.
The diffractive optical surface will in general have positive optical power. Accordingly, unlike the use of a high dispersion negative lens element to achieve color correction, the use of a diffractive optical surface does not require the incorporation of additional positive power into the system to balance added negative power. Indeed, the use of a positive diffractive optical surface can allow for at least some reduction in the power of one or more positive elements in the system which, in turn, can facilitate the overall correction of the system""s aberrations.
The at least one diffractive optical surface can be a blazed kinoform or a binary approximation to a blazed kinoform and can comprise (1) a surface of a separate optical element (e.g., a diffractive optical element (DOE) which is plano on one side and has a diffractive optical surface on the other), or (2) a surface of an element which forms part or all of the positive power lens unit (U2) or the corrector lens unit (UCR).
When formed as part of the positive power lens unit or the corrector unit, the diffractive optical surface provides color correction to the lens system without the need for any additional lens elements. When formed as a surface of a DOE, only one element is required. Accordingly, in either case, the diffractive optical surface of the invention is able to provide color correction for a projection lens system with a minimum increase in the system""s complexity, cost, and weight. Although less preferred, multiple diffractive optical surfaces can be used if desired.